Prior art systems for composting solid waste and sewage sludge typically employ one or more multi-state digesters in which material being treated undergoes staged microbial decomposition. The conventional digester is divided into two or more compartments or stages and during material processing is rotated while air is circulated through the digester at controlled rates under predetermined conditions in a flow direction counter to the material flow. The climate in each stage is maintained to achieve the optimum development of the type and species of microorganism predominant in that stage. Spent air is vented from the digester as needed to maintain optimum climatic conditions in each of the operating stages. Temperatures are kept below 150.degree. F. to ensure the maximum rate of composting consistent with maintenance of the microbial population. Typical of such prior art systems and methodology of operation are those set out and described in U.S. Pat. Nos. 3,245,759 and 3,138,447 assigned to the assignee of the present invention, the teachings of which are hereby incorporated by reference.
The method and apparatus for manufacture of compost described in those patents are designed to produce aerobic decomposition of organic waste materials by maintaining within the apparatus in which the method is carried out conditions suitable for optimum propagation of the different types of aerobic bacteria on which such decomposition depends. The apparatus comprises a digester in the form of a cylindrical drum mounted for rotation on an axis which is slightly declined towards the discharge end relative to the horizontal. The interior of the digester is divided into a series of compartments or chambers by a plurality of transverse partitions spaced along the axis of rotation. Each partition is provided with transfer buckets which are selectively opened and which when opened, transfer material from compartment to compartment from the higher to the lower end of the drum, the raw waste organic material being fed into the digester at the higher end and partially cured compost being withdrawn at the lower end.
An important step in the overall composting process is the final curing of the compost to reduce pathogens after it exits the digester. It is to this phase of the composting process that the present invention is directed.